Internal combustion engines are often provided with a throttle valve within their combustion air induction systems to control combustion airflow rate. The throttle valve is typically in the form of a butterfly valve, having a disk secured to a throttle shaft which is journaled to the generally cylindrical internal wall surface of a throat of a throttle body. The diameter of the disk is only slightly smaller than is the inside diameter of the throat, whereby the throttle plate may block all but a small portion of the air from passing through the throat in an idle condition. Typically, a return spring is used to bias the throttle shaft and butterfly closed with respect to the throat, shutting off almost all the airflow through the throttle valve.
The return springs of such throttle valves are assembled such that they provide a predetermined preload to the throttle shafts. This preload is carefully selected to provide a sufficient force to close the throttle valve when the throttle valve actuating device is released, yet not to provide a force that substantially resists the throttle valve actuating device. Such devices typically include mechanical links, cables or motors attached to an arm extending from the throttle shaft.
Traditional return springs are difficult to assemble quickly and accurately. They are most commonly coil springs that are coiled loosely about the throttle shaft to permit them to be readily tightened or loosened as the throttle shaft is rotated and the valve is opened and closed, respectively. When a throttle valve is assembled, the return spring must first be placed about the throttle shaft. Once it is in position the throttle shaft is then inserted into the throttle body. During this process the free ends of the spring must be twisted about the shaft to apply the proper preload to the spring. While the ends of the spring are being held with the appropriate amount of preload, one end must be transferred to the throttle body and the other end must be transferred to the throttle shaft. In this manner, the preload applied to the spring is transmitted to the throttle body and the throttle shaft. By transferring the ends of the spring to the throttle shaft and throttle body, the preload functions to hold the throttle shaft closed with respect to the throttle body. If a free end of the spring is released prematurely, the spring can rapidly and partially uncoil, causing the preload to be dissipated. This typically requires that the shaft be withdrawn from the throttle body, the spring recoiled, and the shaft reinserted in the throttle body.
What is needed, therefore, is a more efficient method for assembling a throttle valve and an improved throttle valve construction that will permit the throttle valve to be assembled more rapidly and conveniently while reducing the risk that the spring will uncoil.